Impacts of Photoinitiated Chemical Processing on Climate Relevant Aerosol Properties

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Sunlight and atmospheric aerosols are ever-present in our environment. Aerosols (airborne particles or droplets) can impact air quality and climate. Air pollution costs the UK £15 billion/year in damage to human health. The World Health Organization estimates nearly 12% of all deaths worldwide are due to indoor and outdoor air pollution. Aerosols are the largest uncertainty in our understanding of climate. Aerosols interact with sunlight by scattering or absorbing it, which can reduce visibility (e.g. smog) or create a beautiful sunset. However, we have not studied how sunlight can initiate chemistry in aerosols and change the properties of the particles. Recent experiments indicate that light-initiated chemistry may be a common occurrence. However, the impacts of light-initiated chemistry on a range of aerosol properties relevant to climate and air quality are not understood. These reactions may change the identities of the molecules that make up an aerosol particle and may alter fundamental properties including its size and ability to scatter or absorb light. Moreover, this chemistry may produce molecules that partition into the gas phase and can then undergo further chemistry to form new particles.This work will investigate the role of light-initiated chemistry on a range of aerosol properties that are relevant to climate (how much they scatter or absorb sunlight) and to air quality (composition and size). This work will be accomplished using a novel combination of single particle measurements and a photochemical aerosol reactor. In single particle studies, an aerosol particle is captured using an optical trap, irradiated with light of a chosen wavelength, and monitored for changes to the particle's properties. Specifically, the impacts on the size, refractive index, hygroscopic properties, and phase of the particle will be determined. Refractive index determines the ability of the particle to scatter and absorb light. Hygroscopicity determines how a particle's size changes with relative humidity, which also ultimately impacts on refractive index. Phase describes whether the particle is a solid, liquid, or between the two, which can affect how the particle responds to its environment. Additionally, the yield of specific light-induced reactions will be determined at different wavelengths and particle sizes. Further experiments will examine how the surface composition of the particle may impact the chemistry. The photochemical aerosol reactor experiments will allow testing of simulations that scale the single particle measurements to ensemble measurements as well as allow precise elucidation of the molecular pathways operative in the experiments.The results of these experiments will provide a systematic understanding of how light can interact with aerosols to induce chemistry and how that chemistry ultimately impacts climate and air quality relevant particle properties. This systematic understanding will enable predictions of the significance of light induced chemistry on climate and air quality. We then plan to assess some of these impacts by incorporating the newly resolved chemistry into an aerosol chemistry model.